This invention relates to the detection or measurement of attributes of materials generally, for example density, flow rate, moisture content or other attributes or parameters of flowable materials, such as gaseous or liquid fluids, emulsions, particulate or granulate materials or suspensions of solids in liquids, such as a slurry.
A wide variety of devices for such detection and measurement are known in the art. Most of them are specific to a particular attribute or parameter. The general object of the present invention is to provide a reliable and versatile device which can generally be used for a measurement of a variety of different attributes.
The state of the art is represented by GB-A-2222683, which describes a sensor that externally resembles a preferred embodiment of the invention but operates rather differently. The sensor described in GB-A-2222683 comprises an elongate support, preferably in the form of a shaft carrying, preferably near one end, a plurality of laterally extending fins or vanes constituting capacitor plates. The vanes protrude laterally from the support but are generally elongate. The sensor includes circuits responsive to the capacitance between pairs of plates in order to derive an indication of a property, such as moisture content, of the material which is disposed between the plates or fins when the sensor is immersed in that material.
A capacitive technique, like most measurements of moisture, is essentially a measurement of two volumetric ratios. However most definitions of moisture content are by ratio of mass. Although, for example, water density over a wide temperature range is well known, it is also necessary to know the bulk density of the true moisture content by mass to be measured.
The present invention is based on the exploitation of the fact that when a fin or vane is stimulated into small vibrations it has degrees of freedom in two planes, along and across the vane. It obeys a second order rave equation resulting in standing waves set up along the vane. If such a vane is immersed in a flowable material, it is found that the square root of the frequency of oscillation is inversely related to the density of the surrounding medium. However, analysis of wave equation solutions indicates that there is a much greater number of resonant modes and frequencies than are present in, for example, a vibrating tube densitometer. Accordingly, the resonant properties allow exploitation beyond the mere scope of density measurement.
Broadly, according to the invention, an instrument for the detection or measurement of attributes of a material comprises at least one vane which is mounted on a support that allows the vane to vibrate, a drive transducer which is disposed relative to the vane to stimulate vibration therein and at least one sensing transducer disposed to sense said vibrations.
In preferred embodiments of the invention the support is elongate. The vane may comprise a fin which protrudes laterally from the support. However, many other configurations are feasible.
Where the vane is elongate, the said transducers may be spaced apart along the length of the vane. A sensing transducer may be disposed at substantially an antinode of the vibration of the vane. The support may engage the vane at at least one region corresponding to a node of the vibration. In particular, there may be means for clamping the vane near a median region and the drive transducer may be disposed to induce a vibration in a plane transverse the median line.
At least one of the transducers may be disposed on a peninsular portion within the vane. The peninsular portion may be formed as a partly cutout tab which lies in the local plane of the vane. The vane may be clamped between such a peninsular portion and an adjacent part of the vane so that the peninsular portions and adjacent portions each constitute a cantilever. This is useful both for the induction of vibration in and the sensing of vibrations in the vane. The said one transducer may be a drive transducer and the peninsular portion on which the drive transducer is disposed may carry a substantial mass. Where the vane is clamped near the median region, the various transducers may each be disposed on a respective peninsular portion positioned in the median region and spaced apart along it.
By itself a drive transducer might be insufficient to induce vibrations of sufficient amplitude in the vane, and accordingly it is generally desirable to provide a regenerative or positive feedback coupling from the sensing transducer, or one of the sensing transducers, to the drive transducer. This coupling may include a band-pass filter for the rejection of signals other than those associated with a principal or desired mode of vibration of the vane.
As will be explained hereinafter, a useful output may be represented by the output frequency of a sensing transducer or may be obtained by comparing the phase of signals at the drive transducer and one of the sensing transducers or may be obtained by comparison of the amplitude, frequency or phase of the outputs of two sensing transducers disposed at different positions on the vane, for example at antinodes of vibration thereof.